1. Field of the Invention
This invention relates to preserving the electrical and optical properties of optically transparent and conductive films such as indium tin oxide (ITO), and more particularly, to providing a buffer or protective layer between aluminum and ITO for use in the fabrication of flat panel displays and the like.
2. Description of the Related Art
Optically transparent and electrically conductive materials such as indium tin oxide (ITO) find utility in flat panel display (FPD) industries such as field emission displays (FEDs), liquid crystal displays (LCDs), and organic light emitting devices (OLEDs), as well as in solar cells. Surface and bulk characteristics are imperative to the quality of electrical and optical properties of these and other optically transparent and electrically conductive films. It is therefore very important to ensure that such films exhibit the desired surface and bulk properties such that the desired degree of transmission of visible light and electrical properties are obtained.
Devices incorporating ITO often use an aluminum layer deposited over the ITO. For instance, in an FED device where the faceplate is connected to the baseplate using spacers, aluminum is often deposited over the ITO layer in the faceplate to establish sites for the bonding of misaligned spacers. More particularly, an aluminum layer is formed over the ITO layer, the aluminum layer having wells extending therein to the surface of the ITO layer. Bond pads are provided within these wells against the ITO layer at the desired spacer locations. Then, when an array of spacers is brought against the faceplate for anodic bonding, desired spacers contained in the array will bond to the bond pads, while other, misaligned spacers will bond to the aluminum layer. After bonding is complete, the aluminum layer with the misaligned spacers bonded thereto can be removed to leave the desired spacer configuration in the FED.
A problem with using aluminum with ITO in the above and other applications is that ITO is susceptible to corrosion in the presence of aluminum. Atomic and/or ionic diffusion occurs through the aluminum to the ITO during processes such as anodic bonding, thermal cycling, thermal diffusion processes, low energy ion implantation processes, and processes which include electric and/or magnetic fields. ITO is especially susceptible to corrosion in the presence of aluminum when exposed to alkaline or basic solutions or solvents. See, J. E. A. M. van den Meerakker and W. R. ter Veen, J. Electrochem. Soc., vol. 139, no. 2,385 (1992). Corrosion of ITO in alkaline solutions produces SnO.sub.3.sup.2-, which dissolves in the solution, and In metal, which forms grains at the surface. This causes a gray opaque appearance and a disconnection between the ITO and aluminum. Corrosion of the ITO can prove fatal in devices such as flat panel displays by reducing or eliminating the electrical conductivity and optical transparency of the ITO material. This corrosion can also cause delamination of the aluminum layer from the ITO. Redeposition of corrosion byproducts onto the substrate leads to additional defects, e.g., particle defects.
Furthermore, during anodic bonding of spacers to bond pads, excess oxide can change local optical properties of the adjacent ITO between the bond pads. Optical properties may also be changed due to etching.
Accordingly, what is needed is an improved method and apparatus for protecting the electrical and optical properties of an ITO layer and the like when such a layer is exposed to aluminum.